Baseline sensitivities were established for
kasugamycin and
oxytetracycline for 147 strains of Pseudomonas savastanoi pv. savastanoi collected from olive knots throughout California. Minimum inhibitory concentrations for ≥95% growth inhibition ranged from 1.86 to 11.52 and 0.13 to 0.40 µg/ml for
kasugamycin and
oxytetracycline, respectively. In
copper sensitivity evaluations, 95.3% of the strains collected grew at concentrations of metallic
copper equivalent (MCE) of <20 µg/ml, 2.7% grew at MCE between 20 and 30 µg/ml (moderately sensitive), and 2% grew at MCE of 150 µg/ml (resistant).
Copper resistance was never reported previously in the olive knot pathogen, and pathogenicity studies confirmed a high virulence of the
copper-resistant strains. In comparative field studies,
kasugamycin at 200 µg/ml performed equally to the standard
copper hydroxide treatment (MCE of 1,260 µg/ml) for reducing knot development on lateral
wounds of Arbequina and Manzanillo olive inoculated with a
copper-sensitive strain and was better than
copper using a highly
copper-resistant strain.
Oxytetracycline at 200 µg/ml was not as effective as
copper or
kasugamycin but significantly reduced the disease as compared with the untreated control. Field studies on application timings of
copper,
kasugamycin, and
copper-
kasugamycin mixtures to inoculated
wounds indicated that treatments within 24 h of inoculation resulted in higher disease control than applications at later times. In greenhouse trials,
copper or
copper-
kasugamycin applied to
wounds 7 days before inoculation persisted and reduced knot incidence by >50%. Our findings indicate that
kasugamycin is an effective bactericide for controlling olive knot and that the time of any bactericide application after inoculation is critical in managing the disease.